This invention pertains to a device for surrounding an image displayed on a video monitor with a mask that is black or is a selectable shade of gray.
The new mask generator will be described in a situation where it is used to mask an x-ray image displayed on a raster scanned video monitor screen, but it should be understood that the mask generator is applicable to any digital image processing system.
In x-ray fluorographic systems, the original image typically lies within the boundaries of a circle. This results from the x-ray images being received in an image intensifier which converts them to minified and bright optical images that appear on the circular output phosphor of the intensifier. The visual image on the phosphor is viewed with a video camera which converts the image to analog video signals. The analog video signals are converted to digitized picture elements (pixels) and, typically, the digital data is variously processed and reconverted to analog video signals for permitting display of the circular image on the screen of the monitor. It is desirable to have the field outside of the circular image appear dark on the monitor screen in contrast with the image so that accurate diagnostic information residing in subtle differences in pixel intensities can be distinguished.
According to prior practice a black mask was generated by adding an analog signal that is synchronous with power line frequencey to the analog interlaced video signal before digitization. Variations in line frequency caused the interlace to tear, creating a gear artifact on the circular edge of the mask.